High-frequency multiplex signaling system



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. L. ESPENSCHIED HIGH FREQUENCY MULTIPLEX SIGNALING SYSTEM Filed Sept. 30 1919 7 Sheets-Sheet 3 IN VEN TOR.

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July 29 I924.

. 1,502,811 ESPENSCHIED HIGH FREQUENCY MULTIPLE}! SIGNALING SYSTEM I Filed 58D? 50 191? 7 shee'ts sheet 4 I IL 2 @K I) J.

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L. ESPENSCHIED HIGH FREQUENCY MULTIPLEX SIGNALING SYSTEM Filed Sent. 30 1919 '7 Sheets-Sheet 5 IN V EN TOR.

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I-.. ESPENSCHIED HIGH FREQUENCY MULTIPLE)! SIGNALING SYSTEM Filed Sent. 50. 1919 7 Shets-Sheet v INVENTOHRI. Lfispmsczed By Patented July 29, 1924.

UNITED STATES PATENT OFFICE.

LLOYD ESPENSOHIED, OF HOLLIS, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

HIGH-FREQUENCY MULTIPLEX SIGNALING SYSTEM.

Application filed September 30, 1919. Serial No. 3273500.

To all whom it may concern:

Be it known that I, LLOYD EsPENsoHIED,

" residing at Hollis, in the county of Queens and State of New York, have invented certain Improvements in High-Frequency Multiplex Signaling Systems, of which the fol-1 present invention to adapt the terminal circuit arrangementso that the processes of modulation, demodulation and amplification of the various currents in both the transmitting and receiving branches of the terminal circuit may be performed by a single translating device. Although the preferable form of translating device is a single thermionic vacuum tube or a plurality of such tubes in combination adapted to function as a single unit with respect to the two branches of the terminal circuit, yet any well known type of translating device such as the mechanical repeater may be used.

Other objects of this invention will become apparent from the following description when read in connection with the drawings represented by Figures 1 to 14* inclusive. Figure 1 shows two types of ter minal circuits illustrative of the invention, the upper figure showing an arrangement in which the input coils of the translating circuit are in parallel, and the lower figure an arrangement in which the coils are in series. Figure 2 shows a variation of the general circuit arrangement which is distinguishable by the absence of networks for balancing the low frequency and high frequency lines. Figures 3,4, 5 and -6' illustrate certain arrangements whereby this invention may be applied to wireless radio systems asdisi tinguishable from high frequency carrier systems applied to wire lines. Figures 7 and 8 0f he general c cuit arrangement characterized mainly by the positioning of the high frequency and low frequency'filters in the two-wire portions instead of in the four-wire portion of the terminal circuit. Figure 9 is a modincation of the general circuit arrangement characterized by the addition of an amplifier to the translating circuit and by the direct transmission of the carrier frequency per se, which features will be discussed at length hereinafter. The characteristic distinguishing feature of Figure 10 consists in the connection of the high frequency harmonic generator to the artificial line side of the high frequency circuit whereby the carrier current may be distributed to the Various terminal circuits over the regular receiving channels thereby avoiding the use of a special carrier supply circuit. Figure 11 illustrates another modification of the general circuit characterized by the use of a balanced modulator circuit which tends to prevent the circuits from singing over either the high frequency or the low frequency sides, and also having other individual features which will be more fully set forth hereinafter. Figures 12 and 13 illustrate the application of this invention to terminal circuits of a multiplex system wherein the transmission and reception of signals utilizes oppositeside bands of a single carrier frequency. Figure 14; illustrates the application of this invention to a multiplex sys tem for the transmission of telegraph signals, and Figure 145? illustrates a method for generating thecarrier current applicable to the circuit of Figure 14:.

Figure 1 shows two types of terminal circuits of a multiplex signaling system employing a single translating device common to the receiving and the transmitting branches of the terminal circuit. These two terminal circuit arrangements are shown connected to common receiving and transmittting lines in order to avoid unnecessary duplication of the circuit diagram, but it should be understood that, in practice, ter-' minal circuits of different types might be connected with the same lines, or all of the terminal circuits may be of the same type.

In the circuit arrangement shown in the upper part of Figure 1, L represents a line to which any type of low frequency signaling apparatus, as, for example, a telephone transmitt r and receiver, m g be nn ted. A triple winding transformer T of a type which is well known in the telephone repeater art, is adapted to connect the line L, with the terminal circuit. A network N is associated with the transformer for the purpose of balancing the line L The low nary telephone transmission and may be 1 taken, for instance, as from to 2000 cycles.

A transformer T impresses the signaling currents upon the input circuit of the duplex translating device MD,. A transformer T which is also connected with the input side of the translatingdevice is adapt ed-to-impress the high frequency incoming modulated wave upon the t-ranslatingcircuit, Connected with'the output side of the translating device are transformers T and T T being adapted to impress the low frequency demodulated wave upon the receiving circuit 8, and T being adapted to impress the high frequency modulated wave upon the high frequency transmission circuit 9. The two input transformers T, and T, and the output transformer T and T are connected mutually in parallel with respect to the translating circuit. Inserted in the low frequency receiving circuit 8 is a filter "1F, similar to the filter TF in the low frequency transmission circuit 2. Inserted in the high frequency transmission circuit 9 is a filter Tl which is adapted to transmit one of the bands of frequencies based on the carrier frequency, as, for instance, the band of frequencies extending from-the'carrier frequency to that frequency which representsthe upper limit of modulation. This high frequency transmission circuit 9. is connected to the transmission line TL which by means of the triple winding transformer 10 is in turn connected'tothe main transmission line ML. A receiving line RL, which is also connected with the main transmission line ML by means of a transformer 10,. is connected with the inputtransformer T by means of a high frequency receiving circuit 11. This circuit has insertedtherein filter RF, which may be similar to filter TF The duplex translating circuit comprises a pair of thermionic translating devices which are so connected with the translating circuit as to be ableto'modulate and demodulate the frequenciesappliedto the input side of-the devices. Preferable forms of this 'modulating' device are illustrated; in the patent-to B; V. Kendall, No. 1,459 709; dated June 19, 1923, and in the patent to John- E Carson, No. 1,'3A),? ()7 date'd hlune 15, 1920. The conductors 12 connect a high frequency harmonic generator G, with the various terminal circuits and the selection of a proper frequency for the carrier oscilla-' tions of theterminal circuit is determined by the filter 13-Wl1lCl1 is bridged across the circuit 12. An amplifier CA, is inserted in the circuit of the unmodulated carrier current in order to amplify the said current before applying itto the translating device.

This arrangement will be better understood from the following description of its method- .of operation. If the telephone transmitter or other low frequency signaling device associated with said line L, is operated, the low frequency currents arising therefrom will beimpressed upon circuit 2- and will pass through the low frequency filter TF and be applied to the input side of the translating circuit by means ofthe transformerT Theselow frequency oscil-.

lations will, in a manner that is well understood, modulate the high frequency carrier current that is appliedto the commonconductor 14: of the translating device. The resulting modulated high frequency wave will be impressed upon the transformer T, and will pass through the high frequency transmission circuit 9 to the transmission line TL and thence over the main line to the other terminal circuit of the multiplex system. These modulated high frequency oscillations will also be impressed upon the output transformer T of the low frequency receiving circuit 8, but they are prevented from passingback through the low fre quency line L, by means of the low frequency filter RF, which-is so designed as to permit the transfer therethrough of ordinary telephonic frequencieso f, for example, 0 to 2000 cycles. In like manner the bands of frequencies based upon various highfrequency carrier currents will be transmitted over the main transmission line ML and will be impressed upon the common receiving line RL by means of the triple winding transformer 10 and each of the various bands of frequencies will passinto its proper terminal circuit depending upon the characteristics of the high frequencyfilter associated with the terminal circuit. If, for example, the high frequency filter RF associated with the high frequency receiving circuit 11 of the upper diagram of Figure 1. will permit the passage of band of frequencies ranging from 20,000 to 22,000 cycles, the modulated high frequency oscillations based on a carrier frequency of 20,000 cycles will therefore pass into the high fre-' quency receiving circuit 11, and by means of the input transformer beimpressed upon-the input 'side of the duplex tr'anslat- V device. In a manner that is well understood, the high frequency oscillations will be demodulated and low frequency oscilla-r tions will pass through the output transformer T into the low frequency circuit 8, and will be impressed by means of the transformer T upon the line L and consequently upon the low frequency signaling apparatus associated with the said line. Since the transformer T is in parallel with transformer T the low frequency oscillations will likewise be impressed upon the output circuit 9, but owing to the presence of the high frequency filter TF the low frequency oscillations will not pass through and be impressed upon the transmission line TL.

In the circuit arrangement shown in the lower part of Figure 1, L represents a low frequency transmission line which is electrically related to the terminal circuit by means of a triple winding transformer T which is similar to transformer T and is balanced by the network N The low frequency oscillations arising within the circuit L are impressed upon the low frequency transmitting circuit 16 which in turn impresses them upon the circuit 17. A filter F which is adapted to pass only low frequency oscillations is inserted between the circuits 16 and 17. In like manner high frequency modulated oscillations will pass from the common receiving line BL to the high frequency receiving circuit 20, and will be impressed upon theinput circuit 17. A filter F which is adapted to pass only high frequency oscillations, is inserted between the circuit .20 and circuit 17. In the circuit 17 the plurality of coils constituting one of the windings of transformer T are arranged in series, instead of in parallel shown in Figure 2. These series coils eonstitute in effect, a double transformer for the transmission of the high frequency and the low frequency currents from the Circuit 17 to the input side of the translating cir-' cuit. The inner coils of each winding, which are shunted by condensers, constitute the low frequency transformer and the outer coils of each winding together with the condensers constitute the high frequency transformer. In the output side of the translating device we have another transformer T which is adapted to pass the modulated or the dcmodulated currents to the output circuit 23, from which the oscillations will pass into circuits 24 or 25 depending upon whether the oscillations are low frequency or modulated high frequency waves. A low frequency filter F and a high frequency filter F are inserted between the common output circuit 23 and circuits 24: and 25 respectively in order to prevent the passage into the respective circuits of any frequencies except those desired.

The operation of this terminal circuit arrangement is similar to the operation of the circuit arrangement shown in the upper part of Figure 1. Low frequency oscillations arising Wi h n the e reuit n; will be impressed upon circuit 16 and in turn upon circuit 17. By means of the transformer T these low frequency oscillations will be impressed upon the translating device MD and will modulate the high frequency current impressed across the impedance 27 in the common conductor of the translating de ice. This basic caruer frequency is obtained from a common source of carrier oscillations 12 by means of a tuned selecting circuit 28 and is amplified by means of the amplifier CA The modulated high fre quency oscillations will be impressed upon the output circuit 23 by the transformer T and will pass through the high frequency filter F and circuit 25 to the transmission line TL and thence over the main trans mission line ML. If a modulated high fre-' quency wave is received over the main line ML by the receivingcircuit RL, and if the frequencies be within the band of frequencies which the high frequency filter F is adapted to pass, the oscillations will be im pressed upon the input circuit 17. This circuit will in turn by means of the transformer T impress the modulated high freque icy current upon the translating device which will, in a well-known manner, impress the low frequency demodulated wave upon the circuit 23. The low frequency oscillations will pass through the filter F into the circuit 24 and thence into the low frequency line L where they will actuate the low frequency signaling apparatus associated therewith.

It will be seen that by means of the two circuit arrangements shown in Figure 1, the operations of modulation and demodulation are performed by a single translating cir cuit. Themodulating or ordinary telephone frequency is transferred to the modulator input through a low frequency input transformer, thereby modulating the high fre quency carrier current and passing out through the high frequency output transformer as a high frequency modulated cur rent. Current transmitted in the opposite direction passes through a high frequency input transformer is modulated by the carrier current of the same frequency and issues through the low frequency output coil as a telephone current of ordinary frequency. It will be seen that the circuit arrangements shown in this figure are predicated upon the fact that modulation and demodulation are identical although inverse operations and can therefore be effected in the same translating device. The channels which transmit in opposite directions are kept sep arate at the input and output circuit by means of frequency selecting filters, which separation takes advantage of the fact that the frequency is stepped in opposite directions in the sending and receiving channels.

Figure 2 shows a terminal circuit arrange.

&

ment in which no artificial lines are employed to balance the low frequency or high frequency transmission lines. In the circuit arrangement shown in this figure, the low frequency voice currents of line I1 are impressed upon the terminal circuit by means of the transformer T These low frequency oscillations in circuit 2 pass through the low frequency filter TF and are impressed upon the translating circuit by means of the transformer T The translating circuit is preferably that shown in the patent to J. It. Carson previously referred to. The lowfrequency oscilllatio-ns which are impressed upon the circuit modulate the high frequency carrier current and the modulated wave is impressed upon the circuit 5 by means of the transformer T These high frequency modulated waves pass through the high frequency filter TF and are impressed upon the main transmission line ML by means of the transformer T In a similar manner the oscillations which are received over the main transmission line ML pass through circuit 8 and the high frequency receiving filter RF and are impressed upon the translating circuit by means of the trans-- former T These high frequency modulated waves are demodulated in the said circuit by means of the high frequency carrier oscil lations, and the low frequency modulating current is impressed upon circuit 5 by means of the transformer T These low frequency oscillations pass through the low frequency filter RF and are impressed upon the low frequency circuit L by means of the transformer T It will be apparent that the low frequency voice currents transmitted over circuit L may also be impressed upon circuit 3 by means of the transformer T and will pass through the filter RF into the circuit 5. Since, however, the filter TF0 is designed to pass only high frequency oscillations, none of the low frequency oscillations will pass to the main line ML and furthermore, since the low frequency oscillations will be impressed upon the plate side of the translating device, substantially no effect will be produced in the translating circuit. In a similar manner the received high frequency oscillations may pass into both circuits 8 and 9. Those which pass into circuit 9 may be transmitted through the filter TF into the circuit 5, but cannot pass into the low frequency circuit L on account of the presence of the low frequency filter RF and likewise produce no substantial effect upon the translating circuit, since they are impressed upon the plate end of the translating circuit. In the circuit arrangement shown in Figure 2 the amplification through the translating device is limited solely to making up the loss within the terminal circuit itself; that is to say. the

loss through the transmitting path-and the Figures 3, l, 5 and 6 show certain cir-' cuit arrangements by means of which this invention may be applied to wireless sigiialing. In Figure 3 the'transmitter T is connected inductively to an input circuit 2 by means of a t'ransformerS, circuit2 being bridged across the input side of the translating device 5. Impedance coils 4 are connected with circuit 2 to prevent high frequency oscillations from passing to the transmitter T Also assoc ated with the input side of the said translating device is a source of high frequency oscillations 6 which may be any of the well-known type of high frequency oscillation producer. 7 receiver R is connected inductively by means of a transformer 8 with circuit 7 which has therein impedance coils 9 and which is bridged across the output side of the translating device 5. The output circuit 10 of the translating device is loosely coupled by means of the transformer 11 with the circuit 12, which is bridged across the midpoint of two ofthe windings of the triple-winding transformer 13 This triple winding transformer is the well-known type in which the windings are conjugate so that disturbances impressed upon the transformer by one'of the circuits connected therewith will produce no effect upon the circuit which is conjugate to the circuit in which the disturbance arises. Associated with the triple-winding transformer is a network N having associated therewith a variable condenser 0 which jointly are adapted to balance the antenna 14 and its variable condenser C The input coil of the triple-winding transformer and the variable condenser C constitute a resonant cir cuit 15 for oscillations of reception frefrequency carrier current from the high .frequency source 6. The modulated high fre quency oscillations pass from circuit 10 to circuit 12 my means of the loose coupling 11, and are impressed upon the antenna by means of the triple-winding transformer 13. The translating device 5 not only modulates the high frequency carrier current but may also serve to amplify the same. The degree of amplification which can be employed depencs upon the degree of balance between the input and output circuits of the triple winding transformer 13. If these circuits are not properly balanced and the amplification produces a gain that is greater than the loss in the terminal circuit itself, sing ing will take place around the circuit including the translating device and circuits 10, 12, 15 and 16. It will be noted furthermore that in this circuit the high frequency oscillations will be continuously radiated, due to the fact that the modulating device is not of the balanced type in which the hi -h frequency oscillations are suppressed except during the period in which they are being modulated. The high frequency oscillations received by antenna 1 1 will produce oscillations of like frequency in circuit 15 which are transferred by circuit 16 and impressed upon the translating device 5. Since the carrier supply 6 is of substantially the same frequency as received oscillations the said received oscillations will be demodulated by means of the translating device 5 and the low frequency current result ing from demodulation will pass into the circuit 7 and be impressed upon the receiver R The impedance coils 1 and 9 associated with the transmitting and receiving circuits respectively prevent passage of high frequency received oscillations and high frequency transmitted oscillations to the transmitter and receiver respectively. From the foregoing it may be seen that this circuit arrangement provides means whereby a single translating device may produce modulation and demodulation of the transmitted and the received oscillations respectively, and also produce a certain degree of amplification of both sets of oscillations.

In Figure 4 transmitter T is connected by means of the transformer 1 with the common conductor of a duplex-translating device MD one of the windings of the said transformer bein associated with the transmitter circuit and the other winding being connected into the said common conductor. A condenser K is bridged across the winding in the common conductor in order to afford a free passage for the high frequency srillations from the source 9. Bridges across the input side of the duplex device the reactance coil arrangement C This is prefei bly made of two independent coils one designed for high frequency and comprising windings ac and the other for low frequencies and comprising the windings bb. The object of these coils is to permit currents to pass between the common conductor and the two outside conductors in parallel without short circuiting the outside conductors for incoming current from circuit 8. The low frequency retardation coil 6?) may be shunted by condensers K li to facilitate by-passing the high frequency currents from generator 9. In the common conductor of the output side of the said translating device is a transformer 2whereby the modulated carrier oscillations may be impressed upon the output circuit 3. The output circuit is also connected with the receiver R by means of the transformer 1, the coil arrangement of which is similar to that of C One set of low frequency windings is connected with the translating cin cuit and the other winding with the receiver circuit as indicated. The transmitting circuit 3 is connected by means of the circuit 5% across the midpoint of two of the windings of the triple-winding transformer 6. A network N is connected with the trans former 6 in order to balance the antenna circuit 7. A resonant receiving circuit 8 tuned to reception frequency is connected with the input side of the transf rmer 6. The circuit 8 is connected across the coil C in such a manner that the received oscillations will be impressed across the input side of the duplex translating device. The 1. frequency carrier oscillations are impr by a source 9 upon the common conductor of the input side of the translating device.

In the operation of this circuit the low frequency voice currents from the transmitter T are impressed upon the common conductor in the input side ofthe translating device, upon whichare also impressed the high frequency carrier oscillations.

By means of the modulating device the carrier oscillations are modulated by the voice currents and the modulated carrier oscillations are impressed by the transformer 2 upon the output circuit 3, which in turn impresses them upon the circuit 5 and thence upon the antenna 7 from which they are radiated. Oscillations received by antenna 7 are impressed upon the resonant circuit 8, and in turn upon the input side of the translating device across coil C The modulated high frequency oscillations pass into the translating device together with the unmodulated carrier oscillations from source 9 and the former oscillations are demodulated thereby. The low frequency current resulting therefrom is impressed by the transformer 4 upon the receiver R It will be seen that the input connection to the translating device is made transversely across the duplex modulator circuit whereas the output connection is made longitudinally in the common conductor of the plate circuit.

In accordance with this arrangement the input'and output circuits are mutually balanced with respect to the amplifying action of the translating devices, thus preventing high frequency singing around the circuit 2, 3, 5, 8, 0,. It will be apparent therefore, that this arrangement is less susceptible to singing and therefore is more desirable than the arrangement shown in Figure 8 when greater amplification is desired.

In the circuit arrangement shown in Figure 5 the transmitter T is connected with the common conductor of the duplex translating device by means of the transformer 1. A source of high frequency oscillations 2 is connected across the impedance 3 located also in the common conductor l of the duplex translating device. Bridged across the input side ofthe translating device is a coil 0 across which the high frequency received oscillations are impressed upon the translating device. Condensers K, and K in series with the two halves of coil C C respec tively, constitute paths between the gridconductors and the common conductor 4: of the translating device for the carrier frequency. In the common conductor of the output side of the translating device is a coil C across the terminals of which are bridged the con ductors of the output circuit 5. Across the output side of the translatingdevice is a transformer 6 by means of which the low frequency currents resulting from demodulation are impressed upon the receiver The transmitting circuit 5 is connected between the ground side of the antenna and the midpoint of a variable inductance connected in series in the said antenna. This antenna is of the balanced type having a network N, inserted therein which has characteristics simulating those of the said antenna. Condensers K and K, are inserted in the antenna circuit for purposes of tuning. The receiving circuit 7 is bridged across two symmetrical points of the variable inductance and the received oscillations are thereby impressed upon the said circuit and in turn upon the coil C, at the input side of the translating device.

The low frequency .voice currents from the transmitter T,, which are impressed upon the common conductor l, serve to modulate the high frequency oscillations which are impressed upon the same common conductor by the generating device 2. These modulated high frequency oscillations are impressed upon the output circuit by means of the coil C, in the common conductor of the output side of the translating device, and the said oscillations are in turn impressed upon the antenna between the midpoint of the. variable inductance and the ground, and are radiated by the said antenna. The incoming high frequency oscillations to which the antenna is resonant are impressed upon the circuit 7, which in turn impresses them upon the input side of the translating circuit by means of the coil 0 Since the frequency of the oscillations from the generating circuit 2 is the same as that of the incoming oscillations from circuit 7, the received oscillations will be demodulated, and the demodulated low frequency current will be impressed upon the receiver R, by means of the transformer 6. cuit is prevented by the combination of the high frequency antenna balance and the balance between the input and output circuits of the translating devices.

Figure 6 shows another circuit arrangement in which this invention may be employed in radio signaling systems. This circuit is in general similar to that shown in Figure 5, but the distinguishing characteristic consists in the insertion of a vacuum Singing of the cir-' tube oscillator in the antenna circuit, which furnishes high frequency carrier oscillations. One side of the output circuit 5, which in Figure 5 was connected directly with the midpoint of the inductance C is, in the arrangement shown in Figure '6, connected with the filament of the oscillating tube, and thence with the midpoint of the inductance. The'aerial side of the antenna is connected with the grid of this tube, and the side of the antenna which is grounded through the aerial network N, is connected with the plate of said tube. The inductance O together with the inductance and capacity of the antenna to ground, produces a resonant feed back circuit which. will cause the vacuum tube device to act as a producer of high frequency oscillations of the same frequency as that to whichthe antenna is tuned and as the incoming waves. The incoming modulated high frequency oscillations and the unmodulated oscillations of the same frequency, produced by the tube 10, will be impressed upon the circuit 7 and in turn upon the input side of the duplex translating device by means of the coil 0,. The received oscillations will be demodulated by the said translating device, and the low frequency currents resulting therefrom will be impressed upon the receiver R by means of the transformer 6. In like manner the low frequency oscillations from the transmitter T will be im pressed upon the common conductor of the input side of the translating device, and will in turn modulate the high frequency oscillations received from the device 10, producing in the output circuit 5 modulated high frequency waves, which will beimpressed upon the antenna circuit between the midpoint of the inductance C and ground. Since the output circuit is connected between the midpoint of the inductance and ground, and since the network N simulates the characteristics of the antenna,

the input and the output circuits 5 and 7 are mutually balanced with respect to the antenna. Consequently the circuit will not sing, except for the oscillation of the tube 10. A further advantage of this circuit arrangement arists from the location of the high frequency generator within the antenna circuit itself, so that its frequency is controlled by the tuning of the antenna. In this way substantial identity of the frequency of the transmitted and received oscillations may be automatically maintained.

The circuit shown in Figure 6 indicates the manner in which a four-wire terminal circuit may be connected with a two-wire low frequency signaling circuit L The low frequency transmitting and receiving circuits of the four-wire terminal circuit may be connected with the triple winding transformer 11 in the manner shown, the low frequency two-wire circuit L being balance-d by the network N In a similar manner each of the circuits shown in Figures 3, 4, and 5 may be terminated as a two-wire circuit instead of the four-wire circuit as shown in the figures.

if further amplification is desired than that which is given by the translating device shown in Figures 5 and 6, it may be obtained by means of an additional amplifying unit inserted in series with the translating device, and located either in front or behind the same. Such amplifying means are represented symbolically by the rectangles A A A and A 4 shown in Fig. 5 in series with the translating device; the first two are in the input side and the last two in the output side thereof. Or an amplifier such as shown in Fig. 3 of the applicants copending application, Serial No. 461,965, filed April 16, 1921, might be used.

Figure 7 shows a variation of the general type of circuit in which a single translating unit performs both functions of modulation and demodulation for the two sides of the circuit, the variation consisting in the placing of the high frequency and the low frequency filters in the two-wire portions as distinguished from the four-wire portion of the circuit. The circuit arrangement shown in the upper part of this figure represents one of the terminal circuits for the reception or transmission of one of the bands of frequencies based upon a definite carrier frequency, which may be transmitted or received over the main transmission circuit ML. In the circuit arrangement L represents a low frequency signaling circuit, which is connected with the windings of the triple winding transformer T and is balanced by the network N Interposed between the line L and the said transformer, and the network and the said transformer, are low frequency band filters F, and F The high frequency line L is connected with the windings of another triple winding transformer T with which is connected a balancing network N adapted to balance the high frequency line. interposed between the line L and the said transformer, and the network N and the said transformer, are the high frequency band filters F and F, respectively. The midpoints of two of the windings of each of transformer T and T connected with lines L and L,, are connected in parallel, the said connection embracing one of the windings of the transformer T The third windings of the transformers T and T are connected in parallel, the said connection embracing one of the windings of the output transformer T of the translating circuit. Inductances C and C, are inserted in the conductors connecting the said third windings of transformers T and T with the output transformer T in order to prevent the passage of high frequency oscillations between the transformers T and T In the operation of this circuit low frequency voice currents from line L will pass through the low frequency band filter F and will be impressed across the midpoints of the windings of the coil T with which the line L, is connected. These signaling currents will in turn be impressed by the coil 1 of the input transformer T upon the coil 2 of the said transformer, and will accordingly modulate the high frequency oscillations which are impressed upon the translating circuit by the source 3. The modulated high frequency oscillations will pass through the output transformer T, and will be impressed by the winding 4: of transformer T upon the high frequency circuit L The high frequency currents are excluded from the low frequency circuit by inductances C, and C and filters F and F9 The high frequency modulated waves will pass through the high frequency band filter F to the main transmission line ML, and thence to the terminal circuit at the opposite end of the said line. The modulated high frequency oscillations received over the transmission line ML will be conducted by the circuit L through band filter F to the midpoint of the windings of transformer T These oscillations will be impressed across winding 1 of transformer T and will pass through the translating device and be demodulated thereby. The low frequency demodulated current will pass through inductances C and G which offer substantially no impedance to low frequencies, to the winding 6 of transformer T and will be impressed upon the low frequency line L, passing through the band filter F The low frequency currents are excluded from the high frequency circuit by condenser 5 and the high pass filters F F,.

This circuit arrangement illustrates a of the duplex translating circuit.

method by w ich the high frequency and the low frequency currents may be impressed upon a common translating device by means of balancing coils which are connected effectively in parallel. It should be understood that any number of terminal circuits similar to the one described may be connected with the main transmission line, the terminal circuits being made selective by the insertion in each branch high frequency circuit of a band filter similar to F5 but adapted to transmit any desired band of frequencies for the specific terminal circuit. Furthermore a low frequency signaling circuit L might be connected with the transmission line through a suitable low frequency filter F for the transmission of low frequency signaling current over the transmission line simultaneous with the transmission of the various bands of high frequency oscillations.

Figure :8' shows another variation of the general .type of circuit which differs from the circuit arrangement shown in Figure 7, in that the low frequency and the high frequency transformers are connected mutually in series instead of in parallel as shown in Figure? These two transformers may be replaced by one transformer suitably designed to efficiently transmit both high and low frequencies. The coils designated a and a represent the low frequency transformer and the coils designated 6 and Z) represent the high frequency transformer. Another distinguishing characteristic consists in the use of a single balanc' network N adapted to balance the high frequency line which may be connected with a plurality or terminal circuits.

In the figure, L represents a low frequency line, having inserted therein a low frequency band filter F and connected with two ofthe windings of the triple winding transformer T L represents a high frequency branch circuit having inserted therein a high frequency band filter F and con- 'nected with the opposite ends of the same two windings of the transformer T with which the line L is also connected. The midpoints of the said two windings of transformer T are connected with the coil 1 of the input transfori'ncr T the other coil of which is connected across the input side The output side of the said circuit is connected by means of the transformer T with the third winding of the said transformer T Bridged across the circuit L between the filter F and the point of connection of the said circuit with the transformer T is a balancing circuit containing low frequency filter "F and network N which is adapted to balance the low frequency line L Bridged across the circuit L, between the filter F and the point of connection with the windingsof the transformer T is a balancing circuit containing high frequency filter F and the network N which is adapted to balance the main transmission line ML with respect to a plurality of terminal networks.

in the operation of this circuit low frequency currents are received over the line L and will pass through the filter F dand be impressed upon the coil 1 of transformer The high frequency oscillations from the source 3 will be modulated in accordance with the low frequency signaling currents by the duplex translating circuit. The modulated high frequency oscillations will be impressed by means of transformer T upon the third winding of the transformer T and will set up similaroscillations in the circuit containing the other two windings, which will pass th-roughthe high frequencyfilter F over the high frequency circuit L to the main transmission .line ML. These high frequency oscillations cannot pass into thelow frequency line L since the filter F is adapted. to transmit only low frequency currents, In a snnllar manner the received high frequency. oscllla- .tions arriving over the line L will pass through the high frequency band filter F (assuming that they are within that band of frequencies whicht-his filter isladapted to pass) and-will beimpressed across the coil 2 of transformer T These high frequency oscillations will combine with the unmodulated carrier oscillations from the source 3, and will be demodulated in the translating circuit. The low frequency 7 currents derived thereby will be transmitted by the transformer T to the-third winding of the transformer --T and will be. impressed upon the receiving circuit. connected with the other two windings of the transformer. These low frequency currents will pass through the filter F into the low frequency line L and actuate the low fre quency signaling apparatusjassociated there with. These-low frequency oscillations will not pass back into the high frequency circuit L since the. filter F which is interposed betweenthe two circuits,- is not adapted to pass low frequency currents. It will be seen therefore that this circuitarrangement provides means whereby t-hemodulation of the transmitted waves and the demodulation of the received wavesmay'beperformed by a single translating circuit, which is connected with a single two-wire circuit by means of a single triple winding transformer. Furthermore this circuit arrangement shows a. method whereby a single network may be utilized with a plurality of terminal circuits to balance the main trans mission line with which the plurality of terminal circuits may be connected.

Figure 9 shows another variation of: the

general circuit characterized by the inclusion in the translating circuit of an amplifier which is adapted to amplify both the low frequency and the high frequency oscillations that may be impressed upon the translating circuit. A low frequency sig naling circuit L is connected with a termi nal circuitby means of the triple winding transformer T having a network N associated therewith to balance the line L, Associated with the branches of the four-wire circuit are the low frequency band filters F, and F and the high frequency band filters F, and F The output side of the terminal circuit is connected with the branch transmission circuit TL, which is in turn connected with the main transmission line ML by means of the triple winding transformer T The input side of this terminal circuit is connected with the branch receiving circuit BL, which is likewise connected with the main transmission line by means of the transformer T A network N is connected with the transformer T to balance the main transmission line ML. Only one terminal circuit is shown connected with the trans mission line, but it should be understood that any number of terminal circuits may be connected therewith, depending upon the number of different bands of frequencies that may be transmitted over the main transmission line. A low frequency signaling line L having inserted therein a low frequency filter F is connected with the main transmission line in order that low frequency signaling currents may be transmitted simultaneously with the high frequency carrier oscillations. The translating circuit which is bridged across the two-wire sides of the terminal circuit includes transformer T, and amplifier A a second transformer T, a modulating and demodulating device MD and an output transformer T Associated with the modulating and demodulating device Ml) is a source of unmodulated carrier oscillations G.

In the operation of this circuit arrange ment low frequency signaling currents impressed upon circuit L will pass through circuit 1 and be impressed by means of transformer T, upon the amplifier A These amplified. low frequency oscillations then will be impressed by means of the transformer T, upon the device MD there by modulating the high frequency carrier oscillations also impressed upon the device MD, by the generating circuit G. The modulated high frequency oscillations will be impressed by the transformer T upon circuit 2 and will pass through the high frequency band filter F and over the branch circuit TL to the main circuit- ML. Since the filter F is designed to pass only low frequency currents of, for instance, zero to 2,000 cycles, the modulated high frequency oscillations which are impressed. upon circuit 2 which, based on a carrier frequency of, say, 10,000 cycles, may be from 8,000 to 12,000 cycles, will not pass through the filter F into the low frequency line L,. If we assume that the filter F is designed to transmit a band of frequencies ranging from 10,000 to 12,000 cycles, the band of frequencies between 8,000 and 10,000 cycles will be suppressed and only the band included between 10,000 and 12,000 cycles will be transmitted. If the filter F, is designed to transmit the same band of frequencies, oscillations of those frequencies which are received over the main line ML and the branch line BL will pass through the filter F, into the circuit l, and will be impressed upon the amplifier A, by means of the transformer T This amplifier will amplify the modulated carrier current which will then be impressed by the transformer T upon the modulating and demodulating device MD Since the oscillations from the generating circuit G are of the same frequency as the carrier oscillations prior to modulation by voice frequencies, the latter oscillations will be demodulated, and the low frequency modulating current i. e. the voice frequency will be impressed upon the circuit 2 and will pass through the low frequency filter F, to the signaling apparatus associated with the line L Since the modulating and demodulating device MD shown in this Figure is not of the duplex type, such as is shown for example in Figure 1, in which the carrier oscillations are prevented from passing to the output circuit the oscillations of carrier frequency will, in the arrangement shown in Figure 9, be impressed on circuit 2 and transmitted over the branch circuit and the main transmission line ML to the terminal circuit at the other end, which may be assumed to be equipped with a terminal cir cuit similar to that shown in Figure 9. This unmodulated carrier current will pass through the amplifier in the other terminal circuit, and the amplified unmodulatcd carrier frequency may be used therein as the carrier current for the modulation and demodulation of signals in the terminal circuit at the distant end of the transmission circuit ML. This method insures the trans mission of the signaling currents from each end by carrier frequencies which are identically the same in magnitude thereby avoiding the difliculty which arises when slight differences exist between the carrier frequencies transmitted from different ends of the same transmission circuit.

Figure 10 shows another variation of the general circuit having a translating circuit common to the incoming and outgoing paths of the four-wire circuit characterized by the connection of a high frequency harmonic All minal circuit.

generator across the conductors between the network adapted to balance the main transmission line and the triple winding transformer which associates the said line with the terminal circuit. This high frequency harmonic generator may be of any type which is adapted to produce a plurality of high frequency sinusoidal oscillations which may be used as the basic carrier currents for the various signaling channels. In the circuit represented by this figure the low frequency signaling currents of the line L are impressed by means of the transformer T upon the branch 1 of a four-wire ter- These low frequency oscillations pass through the low frequency filter F to the circuit 2 and are impressed upon the translating circuit which is bridged across the two sides of the fourwire circuit. The high frequency oscillations of, for example, 5,000 to 30,000 cycles, which may be generated by the high fre quency harmonic generator G are impressed across two of the windings of the triple winding transformer T and set up oscillations of the same frequencies in the receiving circuit RL and the transmitting circuit TL. If the high frequency band filters F and F are designed to pass a band of frequencies which includes one of these basic carrier frequencies generated by the generator G these oscillations will pass into the circuits 2 and 3 respectively. Those oscillations which pass into circuit 3 produce substantially no effect upon the trans lating device MD since this circuit is connected with the plate side of the translating device. However, the high frequency carrier oscillations which pass through filter F into the circuit 2 are by the trans former T impressed upon the translating circuit together with the low frequency scillations and both are amplified by the amplifier A and modulated by the modulating and demodulating device M13 The modulated high frequency oscillations are then impressed by the transformer T upon the circuit 3 and pass through the high fre quency band filter F to the transmitting circuit TL,' and thence over the main trans mission line ML to the terminal circuit at the other end of this transmission line. The high frequency modulated waves re ceived over the line ML pass into the receiving circuit BL and through the filter F assuming that they are oscillations of those frequencies which the filter F is adapted to transmit. These oscillations then pass into the circuit 2 and are impressed upon the translating circuit by means of the transformer T This band of frequencies is then amplified by the amplifier A and the amplified oscillations are then impressed, upon the modulating and demodulating devic MD by means of the transformer T Simultaneous with the impresslon of this band of received oscillations upon the said modulating and de- 'modulating device, oscillations of the carrier freque cy from the generating source G are likewise being amplified and impressed upon the said device MD The incoming band of oscillations is therefore demodulated by the carrier oscillations of the same frequency and the low frequency modulating current resulting therefrom passes through the transformer T into the circuit 3 and thence through the low frequency filter F into the low frequency line L By this arrangement of the high frequency harmonic generator with the high frequency balancing network and its associated triple winding transformer the basic carrier currents for a plurality of four-wire terminal circuits may be transmitted to each of the said terminal circuits over the reg ular receiving circuit 'RL and through the associated high frequency band filters, thereby avoiding the necessity of providing a special supply circuit for transmitting the said carrier current to the translating circuit associated with each terminal circuit such as, for example, circuit 12 of Figure 1. It should be pointed out that the high'frequency oscillations produced by the generator G will not be transmitted over the main transmission line ML since the conjugate relation between the artificial line N and the real line ML prevents the carrier current from being transmitted, I

Figure 11 illustrates a further modification of the general circuit which is charac terized by the translating circuit common to both the sending and receiving paths of the terminal circuit; by the direct transmission of the carrier frequency; by the balancing of the modulating and demodulating device in order to eliminate amplification components of the various frequencies; by the adaptation of the amplifier of one terminal circuit to operate as an oscillator of carrier frequency and the adaptation of the amplifier of the cooperating terminal circuit at the opposite end of the transmission line to reamplify the oscillations transmitted upon the said first terminal circuit and by the utilization of the principle of multiple balancing whereby a single network may be adapted to balance the transmission line for a plurality of terminal circuits. 7

In the circuit arrangement shown in Fig ure 11, low frequency currents will be received over the line L and passing into the branch 1 and through low frequency filter F of the terminal circuit A will be impressed by means of the transformer T upon the oscillating am alifier A By means of the feed back circuit shown in connection with this amplifier, the device may be made to oscillate at the desired carrier frequency. These carrier oscillations, together with the amplified low frequency currents, will be impressed by means of transformer T upon the modulating and demodulating device MD,and the carrier current will. be modulated accordingly. This type of balanced tube circuit does not permit the passage therethrough of unmodulated carrier oscillations, so that the transformer T, will impress upon the circuit 3 only the bands of modulated frequency which are based upon the carrier frequency, excluding the carrier frequency itself. The high frequency filter F will transmit that band of frequencies which it is designed to pass and the oscillations transmitted will pass over the branch transmission line L and the main transmission line ML to the terminal circuit B at the other end of the said transmission line. The oscillating amplifier A which is producing oscillations of carrier frequency will transmit this frequency through the transformer T into the branch 2 and then through the high frequency band filter F and over the line L and ML to the terminal circuit B. The band of modulated oscillations and the oscillations of carrier frequencywill be simultaneously impressed by means of the transformer T upon the input branch 4 of circuit B and by means of transformer T upon the amplifier A Since this amplifier A has associated with it a feed back circuit which is sharply tuned to carrier frequency, the oscillations of carrier frequency will be fed back, and will be reamplified thereby. These oscillations, together with the amplified oscillations of the band of received oscillations will be impressed by means of transformer T upon the modulating and demodulating device MD The band of received oscillations will be demodulated accordingly and the low frequency currents resulting therefrom will pass through the low frequency filter F into the low frequency line L Low frequency currents set up by signaling apparatus connected with the line L, will be impressed upon branch 5 of the ter minal circuit B, and passing through the filter F will be impressed upon the amplifier A These low frequency oscillations, together with unmodulated high frequency carrier oscillations received from the other terminal A of this circuit, will pass through the said amplifier and will be impressed by the transformer T upon the modulating and demodulating device MD This device is similar to the device MD inthat it will pass the modulated side bands of the carrier fre quency, but Will not transmit the carrier frequency itself. Accordingly the said modulated bands will be impressed by means of the transformer T upon the branch 6 and oscillations of that band which the high frequency band filter F, is adapted to pass, will be transmitted over the branch 7 and the circuits L ML, L to the branch 8 of the terminal circuit A. These oscillations will pass through the high frequency band filter F and will be impressed upon the oscillating amplifier A This band of frequencies will be amplified thereby and together with the oscillations of carrier frequency generated by A will be impressed by means of the transformer T upon the modulating and demodulating device MD,. The band of modulated oscillations will be demodulated and the low frequency current resulting therefrom will pass into the circuit 3, and through the low frequency filter F into the low frequency line L The balanced modulating and demodulating device used in each of the terminal circuits is of a type in which the amplification components of both high and low frequencies are eliminated so that under conditions of substantially perfect balance the device produces no straight amplification. The advantage derived from the use of this type of balanced translating circuit as applied to the present type of terminal circuit is that it prevents the circuit from singing around either the high or low frequency side. Another important feature disclosed by the circuit arrangement shown in this figure consists in the transmission of the unmodulated carrier current from one terminal circuit to another terminal circuit at the opposite end of the main transmission line, and reamplifying the said carrier current at the other terminal so that it may be used therein as the basis of transmission of the low frequency currents from that terminal to the saidfirst mentioned terminal. This insures the maintenance of the same frequencies at each end of the line, and since the demodulation is based on homodyne principles the difficulty which arises when the received frequency and the demodulating frequency are different is thereby avoided.

This circuit arrangement also illustrates another method whereby a single network at each end of the main transmission line may be utilized to balance the said line for a plurality of terminal circuits. In the said circuit arrangement a plurality of triple winding transformers are connected with the branch transmission lines, each transformer being individual to a. terminal circuit and a network N is adapted to balance the main transmission line for all of the terminal circuits associated with the said line. This multiple balancing system may be termed a bridge type, as distinguished from the dif ferential type employing a triple winding transformer common to all terminal circuits, as for example that shown in Figure 1. The advantage of the arrangement shown in Figure 11 arises from the fact that each terminal circuit is made essentially a separate unit which is conducive to flexibility and is slightly more efficient due to the greater efficiency of a transformer that is designed for the particular frequency to which the terminal circuit is adjusted. The terminal circuit B shown at the opposite end of the main transmission line ML is similar to that shown at station A, except that the amplifying de vice A shown at B is designed to reamplify the received carrier oscillations for the purpose of modulation and demodulation at station 13 but the said device A is not intended to generate oscillations for transmission over the-line circuit such as is performed by the oscillation generator A Figure 12 is a modification of the general circuit arrangement differing from that shown in Figure 11 principally'in the use of the opposite side bands of a common carrier frequency for the east and west channels of transmission. Since the frequencies trans mitted and received are different, the filters in the two sides of the circuit are adapted to prevent the passage of a common frequency, consequently the circuit will not sing over either the low or high frequency side, and networks for balancing the low frequency and the high frequency lines may be avoided. In the figure L represents a low frequency line connected with the four-wire circuit having a low frequency band filter F and a high frequency band filter F in the transmitting path of the terminal circuit, and a hi gh frequency band filter F and a low frequency band filter F in the receiving path of the terminal circuit. The input side of the transmitting and receiving paths is connected with the oscillator A by means of the transformer T The output side of the oscillator is connected with a modulating and demodulating device MD by means of the transformer T The output side of the device MD is connected With'the output side of the receiving and transmitting circuit by means of the transformer T Included in the transmitting path is a filter F whose function is to erclude the carrier frequency from the transmitting path of the terminal circuit. The terminal circuit B shown at the opposite end of the main transmission line ML is similar to that described, except ing that the amplifying device A shownin the said circuit is non-oscillating.

This circuit arrangement will be better understood from the following description of its method of operation. Low frequency sig naling currents arriving over the line L pass through the filter F and are impressed upon the oscillating-amplifying device A These oscillations are amplified and together with oscillations of carrier frequency generated in the device A are impressed by the transformer T upon the modulating and de modulating device MD The two bands of only frequencies passing over the main" transmission line ML to the'terminal B at the opposite end of the said line are those contained in the'band which was allowed to pass through the filter F The oscillations of carrier frequency which are generated by the device A will be impressed by the transformer T upon the circuit 3, and pass through the band filter F to the main line ML so that the carrier frequency and one of the transmitted side bands are traveling siiultaneously to the opposite terminal circuit. Both sets of oscillations pass through the high frequency band filter F and are impressed upon the amplifying device A By means of this device both the carrier oscillations and the side band oscillations are amplified and are impressed upon the modulating and demodulating device MD by means of transformer T The oscillations of the side band are demodulated in-the device MD and the low frequency currents resulting therefrom are impressed by means of the transformer T upon the circuit 4. These currents pass through the low frequency band filter F to the low frequency line L Low frequency currents set up in the line L by means of the signaling apparatus associated therewith pass through the low frequency band filter F to the amplifying device A These currents together with the non-modulated carrier oscillations arriving from the terminal circuit at the other end of the side line are impressed simultaneously upon the modulating and demodulating de vice MD and are modulated therein. The modulated high frequency oscillations aris ing therefrom are impressed by means of the transformer T on the circuit 5. The high frequency band filter F is adapted to pass that band offrequencies Which is opposite to the band transmitted by the filter F and to suppress the band of frequencies transmitted by the latter filter. That is to say, if the filter F transmitted the lower band of frequencies based upon the'carrier frequency the filter F is designed to pass the upper band of frequencies based on the same carrier frequency. Filter F is inserted in the transmitting circuit to suppress the carrier frequency. The frequencies transmitted by filter F pass over the transmisv arising within the device A itself are im pressed upon the modulating and demodulating device MD The received band of modulated frequencies are demodulated, and the resulting low frequency currents pass throughthe filter F and the line L to the signaling apparatus associated therewith. The type of modulating and demodulating device represented by MD and MD is adapted to eliminate the amplification components of both the high frequency and the low frequency oscillations so as to prevent singing around the two sides of the terminal circuit. One of the advantages of the circuit arrangement shown in this figure arises from the use of opposite side bands of the carrier frequency whereby balancing net works are rendered unnecessary, and another advantage arises from the transmittal of high frequency carrier oscillations from one terminal circuit to another terminal circuit whereby identity of frequency of the modulating and demodulating current is assured.

Figure 13 shows a circuit arrangement differing slightly from that shown in Figure 12. The principal difference arises in the structure of the translating circuit associated with the four-wire terminal circuit. In the structure shown in Figure 12 the balanced modulating and demodulating circuit is of a type designed to balance out and therefore eliminate the amplification components of both the high frequency and the low frequency currents. The oscillating circuit shown in Figure 18 differs from the foregoing in that it is designed to balance out only the amplification components of the low frequency currents. The carrier cur rent oscillations generated in the device A together with the low frequency currents impressed upon the said device by the transformer T are together impressed by the transformer T upon the modulating and demodulating device MD It will be noted that the windings of the transformer T are adapted so that the middle coil G is coupled with coil G which is connected in the common conductor of the duplex translating device MD The coil C is transversed by the low frequency currents in consequence of which the low frequency currents will be impressed upon the common conductor. The coils C and G transmit primarily the high frequency oscillations so that the said oscillations are impressed across the input side of the translating circuit. The transformer T connected with the output side of the device MD has a plurality of coils connected with the line side which for best results may be connected in the manner shown in the drawing. The high frequency oscillations impressed by the coils C and C upon the input side of the translating device cause corresponding fluctuations in the output side of the said device, which in turn transmit these fluctuations to the coils 4-. and 5, which are connected with the high frequency transmitting branch of the circuit. The low frequency currents which are impressed by means of the coil C upon the common conductor of the translating device produce a flow of current in opposite directions in the output coil of the transformer T so that the low frequency component is balanced out and eliminated. It will thus be seen that the arrangement shown in Figure 13 provides suitable means for balancing out the low frequency component and the arrangement shown in Figure 12 to balance out the amplification components of both frequencies when such is desired.

Figure 14 shows a means for applying this invention to the transmission of telegraphic signals either over wires or by radio signaling system. In the figure L represents a line over which low frequency telegraphic signals may be transmitted to the terminal circuit. This line is connected with the windings of a balanced relay R having associated therewith a network N 1 adapted to balance the line L The armature 1 of the relay is adapted to impress impulses of opposite polarity upon the input circuit 2, which is bridged across a condenser connected into the common conductor of the input side of a duplex translating device. Connected also with the same common conductor, either directly or inductively as shown in the drawing, is a source of oscillations of carrier frequency G. Across the input side of the said translating de ice is a transformer T for impressing high frequency received oscillations upon the said device. Connected with the common terminal of the output side of the device MD 1 is the output circuit 3, by means of which the modulated oscillations may be impressed by the triple winding transformer T 2 upon the transmission line or antenna. work N is associated with the said transformer to balance either the transmission line ML or the antenna. Brid ed across the output side of the translating device MD 1 is a differential relay It 2 the armature of which is adapted to respond to the received impulses and open and close the low frequency circuit L The relay R is so designed that if the network V balances the line L the impulses transmitted over the line L by the operation of the armature 4 will cause no disturbance of the armature 1 in the output side of the relay.

In the operation of this circuit low fre quency tele raphic signals transmitted over the line L will cause the operation of the armature 1 and will apply impulses to the circuit 2 which is bridged across the terminals of the condenser K These 'impulseswill, in a manner well understood, modulate the carrier frequency oscillations A net- 

